45 lines
2.4 KiB
HTML
45 lines
2.4 KiB
HTML
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<HTML><HEAD><TITLE>Built-In Commands - while manual page</TITLE></HEAD><BODY>
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<H3><A NAME="M2">NAME</A></H3>
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while - Execute script repeatedly as long as a condition is met
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<H3><A NAME="M3">SYNOPSIS</A></H3>
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<B>while </B><I>test body</I><BR>
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<H3><A NAME="M4">DESCRIPTION</A></H3>
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The <B>while</B> command evaluates <I>test</I> as an expression
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(in the same way that <B><A HREF="../TkCmd/expr.htm">expr</A></B> evaluates its argument).
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The value of the expression must a proper boolean
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value; if it is a true value
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then <I>body</I> is executed by passing it to the Tcl interpreter.
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Once <I>body</I> has been executed then <I>test</I> is evaluated
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again, and the process repeats until eventually <I>test</I>
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evaluates to a false boolean value. <B><A HREF="../TkCmd/continue.htm">Continue</A></B>
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commands may be executed inside <I>body</I> to terminate the current
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iteration of the loop, and <B><A HREF="../TkCmd/break.htm">break</A></B>
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commands may be executed inside <I>body</I> to cause immediate
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termination of the <B>while</B> command. The <B>while</B> command
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always returns an empty string.
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<P>
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Note: <I>test</I> should almost always be enclosed in braces. If not,
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variable substitutions will be made before the <B>while</B>
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command starts executing, which means that variable changes
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made by the loop body will not be considered in the expression.
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This is likely to result in an infinite loop. If <I>test</I> is
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enclosed in braces, variable substitutions are delayed until the
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expression is evaluated (before
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each loop iteration), so changes in the variables will be visible.
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For an example, try the following script with and without the braces
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around <B>$x<10</B>:
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<PRE>set x 0
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while {$x<10} {
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puts "x is $x"
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incr x
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}</PRE>
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<H3><A NAME="M5">SEE ALSO</A></H3>
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<B><A HREF="../TkCmd/break.htm">break</A></B>, <B><A HREF="../TkCmd/continue.htm">continue</A></B>, <B><A HREF="../TkCmd/for.htm">for</A></B>, <B><A HREF="../TkCmd/foreach.htm">foreach</A></B>
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<H3><A NAME="M6">KEYWORDS</A></H3>
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<A href="../Keywords/B.htm#boolean value">boolean value</A>, <A href="../Keywords/L.htm#loop">loop</A>, <A href="../Keywords/T.htm#test">test</A>, <A href="../Keywords/W.htm#while">while</A>
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<HR><PRE>
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<A HREF="../copyright.htm">Copyright</A> © 1993 The Regents of the University of California.
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<A HREF="../copyright.htm">Copyright</A> © 1994-1997 Sun Microsystems, Inc.
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<A HREF="../copyright.htm">Copyright</A> © 1995-1997 Roger E. Critchlow Jr.</PRE>
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</BODY></HTML>
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